1.Field of the Invention
This invention relates to an optical wavelength meter.
2.Description of Related Art
Interferometer optical wavelength meters are known for measuring wavelengths in the optical C-band (1525-1565 nm) and optical L-band (1568-1610 nm) at a rate of approximately 10 measurements/sec. The use of linear and periodic optical filters for wavelength measurement is known from laser characterisation measurements. However, it is desirable to make faster optical wavelength measurements within these wavebands, for example, in VCSEL (vertical cavity surface emitter laser) production wafers having 20,000 devices need to be tested and in mass production of other active or passive optical components.
Referring to FIGS. 1 to 3, the measurement of optical wavelength using coarse and fine wavelength dependent filters is known from, for example, GB 0223448.2 for characterisation of tuneable lasers. A known arrangement 10 is shown in FIG. 1, in which a laser beam 11 from the tuneable laser 12 is split by a branched waveguide to direct the light beam under test to four pathways 131,132,133,134 leading respectively to: a reference photodiode 141; a coarse, monotonic or linear spectral filter 15 for the wavelength band of interest with an output to a second photodiode 151; and two periodic transmission filters 16,17, the respective outputs of which vary periodically with wavelength across free spectral ranges, having outputs to third and fourth photodiodes 161,171, respectively. Typically the periodic filters 16,17 are Fabry Perot etalon filters having 50 GHz and 5 GHz free spectral range respectively, as shown in FIG. 2.
The course feature extraction filter 15 is a dielectric multilayer coating on a glass substrate with a monotonic frequency response across The C-band; while the medium and fine frequency identification filters 16, 17 are, for example, each formed from two parallel 95% dielectric minors with flat response.
Characteristics of the filters are shown in FIG. 2, in which a first plot 21 shows a linear response of the coarse filter 15, a second plot 22 shows a periodic response of the first periodic filter 16 with a free spectral range of 50 GHz and a third plot 23 shows a periodic response of the second periodic filter 17 with a free spectral range of 5 GHz.
In use, the wavelength of an emitted beam is determined approximately with the linear filter 15 by determining transmissivity of the filter at the given wavelength by comparing the power of the reference beam, as measured with the first reference photodiode 141, with the power of the beam transmitted through the filter 15 as measured by the second photodiode 151. The wavelength is determined with sufficient precision, e.g. is found to be in the range 24 shown in FIG. 2, to determine on which peaks 221, 231 of the periodic filter transmission curves 22, 23 the measured wavelength lies. As shown in FIG. 3, knowing on which peaks the wavelength lies, the wavelength 241 may be determined unambiguously from the outputs of the third and fourth photodiodes 161, 171.
It is an object of the present invention at least to ameliorate the aforesaid deficiency in the prior art.